1. Field of the Invention
The present invention relates to rechargeable galvanic cells comprising a divalent cation conducting solid electrolyte in contact with a positive electrode, and a solid-state negative electrode contacting a molten salt providing ionic mediation to the solid electrolyte. Galvanic cells of this type may be assembled to form advanced storage battery systems for use in utility load leveling, electric vehicle propulsion, and aerospace power source applications.
2. Description of the Prior Art
Both ambient and high temperature battery systems are currently being considered for applications such as electric vehicle propulsion and utility load leveling management. Ambient temperature batteries are typified by aqueous electrolyte systems such as lead-acid, zinc-chlorine, zinc-bromine and nickel-zinc. Also, there has been increasing interest in secondary lithium non-aqueous systems such as Li/TiS.sub.2 and and Li/CuCl.sub.2. Ambient temperature aqueous electrolyte systems generally demonstrate faster electrode kinetics, and can deliver higher power densities than lithium non-aqueous systems, although they generally possess lower volumetric energy densities.
The discovery that sodium ion conductivity in sodium beta-alumina and beta"-alumina type materials at room temperature is comparable to conductivity in aqueous solutions greatly enhances the applications of high conductivity solid electrolytes. Most work has focused on the high cationic conductivity in solids conducting monovalent ions, but it is now known that ion exchange reactions result in replacement of monovalent sodium ions in sodium beta"-alumina with divalent cations, such as Ca.sup.2+, Sr.sup.2+, Ba.sup.2+, Zn.sup.2+, Cd.sup.2+, Pb.sup.2+ , Hg.sup.2+ and Mn.sup.2+. Most of the divalent beta"-aluminas demonstrate high divalent cationic conductivities of about 10.sup.-1 (ohm-cm).sup.-1 at temperatures from about 300.degree. to 400.degree. C. for single crystal materials, suggesting their suitability for use in high temperature secondary cell applications. G. C. Farrington and B. Dunn, "Divalent Beta" Aluminas: High Conductivity Solid Electrolytes for Divalent Cations", Mat. Res. Bull., 15:1773 (1980). The ionic conductivities of Ca.sup.2+, Sr.sup.2+ and Ba.sup.2+ beta"-aluminas are comparable, while the conductivity of Pb.sup.2+ beta"-alumina is greater and approaches the conductivity of sodium beta"-alumina at temperatures below 25.degree. C. R. Seevers, J. DeNuzzio, and G. C. Farrington, "Ion Transport in Ca.sup.2+, Sr.sup.2+, Ba.sup.2+ and Pb.sup.2+ Beta"Aluminas", Office of Naval Research Contract NOOO14-81-K-0526, Technical Report No. 2 (1983). Another report relates the structure of divalent beta"-aluminas to their conductivity. J. O. Thomas, M. Alden, and G. C. Farrington, "The Relationship Betweeen Structure and Ionic Conductivity in Divalent .beta."-Aluminas", Solid State Ionics 9 & 10: 301 (1983). Divalent cations are typically substituted for sodium in beta"-alumina by immersion in an appropriate molten salt of the desired divalent cation. E. E. Hellstrom and R. E. Benner, "Preparation and Properties of Polycrystalline Divalent-Cation Beta"-Alumina", Solid State Ionics 11: 125 (1983). Divalent beta"-aluminas are generally stable, they retain the beta"-alumina structure, and they can be reversibly exchanged from sodium beta"-alumina.
Trivalent cations, including Gd.sup.3+, N.sup.3+ and Eu.sup.3+ may also be substantially completely exchanged for sodium ions in beta"-alumina single crystals to provide solid electrolytes having rapid trivalent cation mobility at moderate temperatures. B. Dunn and G. C. Farrington, "Trivalent Ion Exchange In Beta" Alumina", Office of Naval Research Contract NOOO14-81-K-0526, Technical Report No. 6 (1984). The ionic distribution in trivalent Gd.sup.3+ beta"-alumina, characterized by X-ray diffraction study, indicates that these materials may have interesting optical properties, although ionic conductivity is relatively low. W. Carrillo-Cabrera, J. O. Thomas and G. C. Farrington, "The Ionic Distribution in Trivalent Gd.sup.3+ Beta"-Alumina", Solid State Ionics 9 & 10:245 (1983).
Recent experimental developments in high temperature galvanic cells have focused primarily on sodium-sulfur battery systems and lithium alloy-metal sulfide battery systems. The sodium sulfur cells comprise a sodium beta-alumina type electrolyte, highly reactive liquid sodium negative electroactive material, and sodium polysulfide positive electroactive materials. The lithium alloy-nickel sulfide systems currently being developed utilize a molten salt electrolyte. These cell systems typically operate at 350.degree. C. and 450.degree. C., respectively, to promote rapid electrode kinetics, thereby achieving high discharge power densities. The high temperature operation of these cells has been determined by the melting point of the sodium polysulfide oxidant at the cathode of the sodium-sulfur cell and the melting point of the LiCl-KCl molten salt electrolyte used in the lithium alloy-metal sulfide battery. High energy densities for these types of cells are expected since alkali metals are used for the negative electrode in both of the systems.
The sodium-sulfur system is considered by many researchers to be the most promising of the advanced high temperature battery systems. The sodium ion conductivity of sodium beta-alumina type solid electrolyte material allows effective separation to be realized by the highly reactive liquid sodium negative electroactive material and the sodium polysulfide positive electroactive material. One important drawback of this type of system is the containability of molten sodium at 350.degree. C. in the event of a cell leak or accident. In addition, there is some evidence of material degradation of sodium ion conducting beta"-alumina ceramic electrolyte at the negative electrode side upon extended cell cycling, which may lead to cell shorting.
Galvanic cells utilizing iron-doped beta-alumina type cathodes are operable at closer to ambient temperatures than conventional sodium-sulfur beta-alumina systems. Cells comprising sodium negative electrodes, beta-alumina ceramic electrolyte, and sintered beta-alumina positive electrodes in which some of the aluminum sites were replaced by iron in the beta-alumina structure operated at about 120.degree. C. were shown to be electrochemically regenerative. "Galvanic Cells Containing Cathodes of Iron-Doped Beta-Alumina", J. H. Kennedy and A. F. Sammells, J. Electrochem. Soc. 121:1 (1974).